1. Field of the Invention
The present invention relates to a circuit for detecting a low voltage, and also to a current-limiting circuit for limiting the output current of a power device when the low voltages rises above a predetermined value.
2. Description of the Related Art
FIG. 1 shows a power device T.sub.1 designed to function as a main switch, and also a current-limiting circuit 11 for limiting the current flowing through the power device T.sub.1. The current-limiting circuit 11 comprises a lateral NPN transistor Q.sub.1 and two resistors R.sub.1 and R.sub.2 (e.g., diffusion resistors, polysilicon resistors, or aluminum wiring). Advantageously, this circuit 11 can be formed on a chip, by a method substantially identical to the method of manufacturing a discrete power transistor.
The current-limiting circuit 11 can restrict the output current I.sub.O of the power device T.sub.1. More precisely, when the output current I.sub.O increases over the value I.sub.OL given below, the transistor Q.sub.1 is turned on, thereby limiting the output current I.sub.O of the power device T.sub.1. ##EQU1##
To limit the output current I.sub.O of the device T.sub.1, the voltage drop made by the resistor R.sub.2 must be greater than the voltage V.sub.BEQ1 which turns on the transistor Q.sub.1 . Inevitably, the current-limiting circuit 11 has a great power loss P.sub.D, which is given as follows: EQU P.sub.D =I.sub.OL .times.V.sub.BEQ1 =I.sub.OL.sup.2 .times.R.sub.2( 2)
In practice, not only the power loss P.sub.D, but also the power loss P.sub.D ' should be considered, which is made by the resistance R.sub.DS existing between the drain and source of the power device T.sub.1. The total power loss P.sub.DT in the circuit comprised of the device T.sub.1 and the circuit 11 is, therefore, considerably large, as can be understood from the following equation (3). ##EQU2## where V.sub.DS(ON) is the drain-source voltage of the power device T.sub.1.
FIG. 2 shows a power device T.sub.1 designed to function as a main switch and developed to solve the problem with the circuit shown in FIG. 1, and also a current-limiting circuit 11 identical to the circuit 11 shown in FIG. 1, used for limiting the current flowing through the power device T.sub.1. As is evident from FIG. 2, the power device T.sub.1 comprises a plurality of small cells. The small cells are divided into two groups, the first consisting of n times as many cells as the the second group, where n is a number greater than 1. Hence, the output current I.sub.O of the device T.sub.1 is divided into two parts having magnitudes n and 1, respectively. Hence, the current I.sub.O ', defined below, is supplied to the resistor R.sub.2. ##EQU3##
In the circuit shown in FIG. 2, when the output current I.sub.O of the power device T.sub.1 increases above the value I.sub.OL defined by equation (5), the transistor Q.sub.1 is turned on, whereby the current I.sub.O is limited. ##EQU4##
If n&gt;&gt;1, the power loss PD made by the resistor R.sub.2 can be neglected. Therefore, it suffices to consider the power loss P.sub.D ' resulting from the drain-source resistor R.sub.DS of the power device T.sub.1. Hence, the total power loss P.sub.DT in the circuit comprised of the device T.sub.1 and the circuit 11 is less than that in the circuit of FIG. 1. EQU P.sub.DT =P.sub.D '=I.sub.OL .times.V.sub.DS(ON) =I.sub.OL.sup.2 .times.R.sub.DS ( 6)
To cause a current to flow through the power device T.sub.1, the following relation (7) must avail: EQU V.sub.DS(ON) &gt;V.sub.BEQ1 ( 7)
The total power loss P.sub.D in the circuit of FIG. 2 is greater than I.sub.L V.sub.BEQ1 as can be understood from the following equation: EQU P.sub.DT =I.sub.OL .times.V.sub.DS(ON) &gt;I.sub.OL .times.V.sub.BEQ1( 8)
In either conventional current-limiting circuit 11 described above, use is made of the base-emitter voltage V.sub.BEQ1 (about 0.6 V) of the transistor Q.sub.1 to detect the output current I.sub.O of the power device T.sub.1. The power loss P.sub.DT of the circuit comprising the device T.sub.1 and the circuit 11 is inevitably large.